Reproduction steps:
1. Open the attached project "TimelineSignal.zip"
2. In the Project window under "Scenes" open "SampleScene.unity"
3. Enter Play Mode
4. The Signal track will activate and a message is printed out in the Console window
5. Press the Button in the Game view to see the same message being printed out again

Expected result: Timeline Signal does not get called twice when the Signal is in frame 0
Actual result: Timeline Signal gets called twice when the Signal is in frame 0 (See attached TimelineSignal.mp4)

Reproduces on: Timeline 1.2.0 (2019.3.0a1), Timeline preview.2 - 1.5.0 (2019.4.2f1, 2020.1.0b14, 2020.2.0a16)
Could not test 2018.4 (No Signal Track)

Notes:
-Issue is still present if Emit Once property is checked
-Issue does not reproduce if Signal gets called in frame 1 and above